This invention relates generally to the field of telephone protection modules associated with individual telephone subscriber lines, for the joint purpose of shielding such lines from damage caused by the presence of excessive voltage or excessive currents which may result from contact with fallen power lines, incorrect connections with non-telephone circuits, and the like.
Depending upon the particular requirements of individual telephone systems, protector modules will normally include a pair of carbon blocks defining a gap over which excessive voltages may arc in the process of being conducted to a source of ground potential. The carbon blocks are often combined with heat-senstive means responsive to the presence of excessive currents to permit a resilient means to short said currents through the carbon blocks to ground potential. Other types include a more recently developed gaseous discharge device, the outer surfaces of which are employed to conduct the currents to ground potential.
While such constructions offer protection to the particular subscriber line with which they are associated, the above described mode of operation often results in unnecessary damage to the module or the components thereof, whereby repair of the same for reuse is no longer practical. Where excessive currents are allowed to arc through the carbon blocks or gaseous discharge device, the heat developed by such currents, often in the order of 100 amperes or more results in a welding action destroying both the casing of the module and the internal components thereof. Prior art devices have, in the past, been considered expendable, but the cost of manufacture of more recent sophisticated designs is no longer inconsequential.